The present invention relates generally to interface units on digital transmission line systems and, more particularly, to a Network Interface Unit ("NIU") that may report its status to a remote facility. Thus, for example, the invention assists a telephone company technician in identifying, from a central office, the location of a break in a digital transmission line.
NIUs are typically located at the interface between a digital transmission line and a customer premises and usually serve as the demarcation between a telephone company line and a customer premises. In general operation, when an NIU receives a signal on the digital transmission line from the central office, the NIU, in turn, transmits that signal to equipment on the customer premises. Similarly, when the NIU receives a signal from the customer premises, the NIU transmits that signal to the central office on the digital transmission line.
The present invention may be used with digital transmission lines generally, including, for example, the Regional Bell Telephone Systems in the United States. As is discussed in Pesetski and Arnone U.S. patent application Ser. No. 07/943,859, filed Sep. 11, 1992, the Bell Telephone System has widely utilized time multiplexed pulse code modulation systems. Such systems have generally been designated as "T carriers." The first generation of multiplexers designed to feed the T1 system was the D1 channel bank. Channel banks have evolved through the D5 series. The "D" channel bank provides multiple DS-1 signals that are carried on the T1 systems. Each T1 system carries twenty-four two-way channels on two pairs of exchange grade cables. One pair of cables provides communication in each direction. The information on such a pulse code modulated system is transmitted in the form of bipolar or alternate mark inversion (AMI) pulses.
The data to be transmitted over the cables, such as speech, may be sampled at a rate of 8,000 Hertz, and the amplitude of each signal is measured. The amplitude of each sample is compared to a scale of discrete values and assigned a numeric value. Each discrete value is then encoded into a binary form. Representative binary pulses appear on the transmission lines.
As discussed in Sheets U.S. patent application Ser. No. 07/844,129 filed Mar. 2, 1992, the data, or "payload," signals on such transmission lines are typically sent differentially on a Tip-Ring pair. Payload signals are received by the telephone company central office and, generally, are transmitted, via cables, to a series of regenerative signal repeaters ("line repeater" or "signal repeater"). Such repeaters are spaced along the cables approximately every 6,000 feet. The first repeater receives the data from the central office, but, because of transmission line losses, noise, interference, and distortion, the signal will have degenerated. The repeater recognizes the presence or absence of a pulse at a particular point in time and, thereafter, if appropriate, regenerates, or "builds up," a clean, new pulse. A regenerative repeater is powered by the transmission cable itself to generate the new pulses. The new pulses are transmitted by the line repeater along more cable to either another line repeater or to an NIU.
As further explained in Sheets U.S. patent application Ser. No. 07/844,129 filed Mar. 2, 1992, some "intelligent" line repeaters also include a "dead loop" feature. In this mode, a break in the transmission line or a disconnection of the customer's equipment from the NIU causes the line repeater or the NIU to "dead loop," such that any signal transmitted from the central office is simply rerouted back to the central office. Accordingly, the central office is advised of the abnormality along the transmission cables. The "dead loop" condition may be released if, for example, the line is corrected or the customer's equipment is reconnected to the NIU.
As further noted in Sheets U.S. patent application Ser. No. 07/844,129 filed Mar. 2, 1992, NIUs commonly have the capability to identify errors in the data received over the cable and responsively provide a signal to the central office that the errors have occurred. Errors that can be detected by the NIU include, for example, errors in signaling, format, bipolar violations, out of frame data, or loss of signal, as well as the disconnection of equipment by the customer.
NIUs may include regeneration toward the customer premises. Similarly, the NIU may include regeneration in the opposite direction. The NIU may also include analog to digital circuitry to convert an analog signal from the customer premises to a digital signal for the central office.
For clarification and simplification of terminology, the pair of cables carrying signals from the central office to the NIU is designated as a "network receive" line, and the pair of cables transmitting data from the NIU is designated as the "network transmit" line. These designations are conventions in telecommunications defining directions relative to the end customer. Similarly, the two pairs of lines carrying signals to and from the NIU to the customer premises is designated as the "customer premises" lines.
NIUs may also include a "loopback" feature that allows the central office to ascertain whether or not a particular span of cable provides continuity along its entire length. For example, the central office may send, via the digital transmission lines, an activating signal, which does not interfere with normal transmission operations, that designates the NIU to "loop back" a signal to the central office. If no break is present on transmission lines, the NIU will receive the signal on the network receive line, and transmit a loopback signal to the central office on the network transmit line.
If, however, the digital transmission line has a break on either the network receive line, network transmit line, or both, then the central office will not receive a loopback signal in response to an activating signal. That is, if a break is present on the network receive line, the NIU will never receive an activating signal, or if a break is present on the transmit line, the central office will never receive a loopback signal on the network transmit line. Use of a loopback feature with repeaters is discussed in Garcia U.S. Pat. No. 5,224,149, issued Jun. 29, 1993.
In some instances, the activating signal for an NIU loopback becomes distorted, interrupted, or lost. In addition, false loopback signals may render the loopback procedure worthless for identifying the location of breaks in transmission lines.
These types of problems may occur because the loopback circuitry is coupled not only to the network receive lines, but also to the customer premises lines. Thus, a signal on the customer premises line may interfere with the loopback procedure. For example, a voltage on the customer premises lines may cause the NIU to transmit an unprompted loopback signal to the central office. Similarly, a voltage on the customer premises line may interfere with loopback circuitry and disrupt the loopback procedure.
One source of voltage on the customer premises line is open circuit reflection. An open circuit may appear on the customer premises line if, for example, lines are in the process of being installed in the customer premises, a customer disconnects equipment from the NIU, or there is a broken line within the customer premises. If the customer premises lines are an open circuit, open circuit reflection may occur, for example, if the length of the open circuit customer premises line is equal to one fourth of the wavelength of the signal from the central office. Similarly, where the length of the customer premises line is equal to the wavelength of the signal from the central office divided by four and multiplied by (2n+1), where n is a positive integer, open circuit reflection may cause a voltage null on the input of the sensing regenerator in the loopback circuitry.
One past method of addressing the problem of open circuit reflection has been to couple a high output impedance amplifier on the line from the NIU to the customer premises. This method is not desirable because it consumes current and is a power drain, and incurs the expense of a high output impedance amplifier in each NIU. Worse still, with the loss of local power, the output amplifier would not function and the transmission would be blocked in the receive direction.
Many of the concepts discussed in this background are further explained in Tarbos U.S. Pat. No. 3,568,100, issued Mar. 2, 1971, Garcia U.S. Pat. No. 5,224,149, issued Jun. 29, 1993, Pesetski and Arnone U.S. patent application Ser. No. 07/943,859, filed Sep. 11, 1992, and Sheets U.S. patent application Ser. No. 07/844,129, filed Mar. 2, 1992.